A computer system that classifies or searches documents containing texts in natural languages or graphics based on the meanings conveyed by the texts or graphics can be useful in many areas. The texts may include numerals, formulas or other suitable forms. The graphics may include drawings, photos, diagrams, charts or other suitable forms. However, neither natural languages nor graphics are completely accurate. For example, one word may have multiple meanings; multiple words may have the same meaning; and the meaning of a word, phrase, sentence or graphic may be affected by the context or the tone of the author. Further, the meanings conveyed by a document may depend on information extrinsic to the document itself. For example, a reader's prior experience or mood may affect perception of graphics and texts in a document. The inaccuracy of natural languages and graphics or the effect of extrinsic information may lead to errors of the computer system. The errors may cause significant legal or financial risks to a user of the computer system.